It is known to admix particles of water-insoluble hydrogels, so-called superabsorbents, in fibre structures for absorption purposes, for instance absorbent bodies in diapers, sanitary napkins, incontinence guards, wound dressings etc. These superabsorbents are polymeric materials that are capable of absorbing large quantities of liquid. One problem encountered when mixing superabsorbents in fibre structures is to fix the superabsorbent particles in the structure so as to localize the superabsorbent in the correct position in the product, and so that the product can be handled in manufacture and during transportation without the superabsorbent being redistributed in the product or shaken from the fibre structure. Dusting caused by fine-grain superabsorbents can also lead to an environmental problem at the place of manufacture.
Several methods of adhering superabsorbent to fibres are known to the art. According to one method described in U.S. Pat. No. 4,354,487 and EP-A-0,402,650, a monomeric solution is allowed to infiltrate a fibre network or is mixed with free fibres. The monomers are then allowed to polymerize in place on the fibres. Prior publication U.S. Pat. No. 4,888,238 describes a method in which dissolved polyelectrolytes form complex bonds on the fibres. According to another method, described in U.S. Pat. No. 4,128,692, fibres are first suspended in water and the superabsorbent is then added and precipitates onto the fibres. In all of the aforesaid cases it is necessary to evaporate-off the solvent, i.e. the water, in one way or another, after having bound the superabsorbent to the fibres. This is difficult to achieve when the water is bonded firmly to the superabsorbent.
According to another method, described in U.S. Pat. No. 4,721,647, a water-soluble monomer having double bonds is allowed to form droplets around a hydrophobic fibre, these droplets being allowed to polymerize and form cross-links. The underlying concept is that the superabsorbent shall embrace the fibre so as not to be released when absorbing liquid. According to the measurements reported in said patent, less than 20% by weight of the superabsorbents are released in the process of absorption. It is also mentioned in the patent that the absorbent droplets should not be applied too densely, since this would stiffen the fibres.
Another method of adhering superabsorbents to fibres is described in WO 90/11181 and WO 91/10413, according to which fibres are coated with a binder which binds to superabsorbent particles. The binder may consist of an ethylene vinyl acetate copolymer, ethylene acrylic acid copolymer, polyvinyl chloride or some other thermosetting resin, for instance. The binder is sprayed onto the fibres a number of times, so as to effectively cover the fibre surfaces. Absorbent grains are then applied to the fibres before the binder has set. Between 15 and 50 % by weight of the absorbents, based on the dry fibre weight, are bonded when practicing this method, which is about equal to the amount of superabsorbents that is effectively bonded when practicing the other methods aforesaid.
U.S. Pat. No. 5,002,814 discloses another solution to the problem, in which a particle of a superabsorbent polymer has been bound to several short fibres.